The present invention relates to a process for producing an aromatic polycarbonate and an aromatic polycarbonate composition, and more particularly, to a process for producing a transparent and colorless aromatic polycarbonate having a high quality, at a high polymerization velocity without occurrence of significant foaming phenomenon, by the transesterification reaction of an aromatic diol compound and a carbonic acid diester, especially such a polycarbonate for optical applications, and a transparent and colorless aromatic polycarbonate composition having a high quality.
Recently, aromatic polycarbonates have been extensively used as engineering plastics in various applications such as office automation (OA) parts, automobile parts, building materials and the like, because these polycarbonates are excellent in not only mechanical properties such as impact resistance but also heat resistance, transparency and the like. Especially, these aromatic polycarbonates have been widely applied to optical materials such as lens, discs or sheets, due to excellent impact resistance and transparency thereof.
The aromatic polycarbonates have been industrially produced by a socalled phosgene method of reacting an aromatic diol with phosgene by interfacial polycondensation method. However, in the phosgene method, it is inevitably required to use phosgene which is quite harmful to human bodies, as well as a solvent such as dichloromethane adversely affecting environmental conditions. In addition, in the phosgene method, a large amount of sodium chloride as by-product is mixed in the aimed polymer, thereby causing problems such as corrosion when the polymer is applied to electronic parts.
Also, there is hitherto known a so-called melting method or non-phosgene method in which the aromatic polycarbonate is produced by subjecting an aromatic diol compound and a carbonic acid diester both kept in a molten state to transesterification while discharging low-molecular weight compounds produced as by-products such as phenol out of the reaction system. The non-phosgene method has such an advantage that the aromatic polycarbonate can be produced without suffering from such problems as observed in the above interfacial polycondensation method. However, the non-phosgene method should be conducted at a higher polymerization temperature as compared to that of the phosgene method, so that the obtained polycarbonate product tends to undergo undesired discoloration. Further, in the non-phosgene method, it is required to continuously increase a molecular weight of the polycarbonate while discharging the by-produced low-molecular weight compounds such as phenol from the reaction system, thereby causing the increase in viscosity of the reaction system with the passage of time. This leads to occurrence of severe foaming phenomenon due to evaporation of the low-molecular weight compounds, thereby inducing entrainment of splashes or rapid rise of the liquid level which in turn cause not only clogging of exhaust systems but also such a problem that degraded materials attached to an upper portion of the polymerization reactor are mixed in the aimed product, resulting in discoloration and poor quality of the product. When the exhaust velocity is lowered in order to inhibit the foaming phenomenon, the polymerization velocity becomes insufficient, so that the obtained polymer is exposed to an elevated temperature for a longer period of time, resulting in deteriorated color tone of the polycarbonate produced. Besides, in order to increase the exhaust velocity to such an extent sufficient to reduce a residence time in a heated state of the polycarbonate, it is necessary to use a polymerization reactor having a far larger capacity than the volume of liquid to be treated, resulting in inefficient process from the standpoints of heat efficiency and productivity.
As the method of inhibiting the above foaming phenomenon, for example, in Japanese Patent Application Laid-Open (KOKAI) No. 9-286850(1997), there is described a method using so-called defoaming blades for directly agitating the liquid level and mechanically breaking foams produced. However, the above conventional method has problems such as limited effect due to the mechanical defoaming operation, and use of excessively large-scale facilities.
As a result of the present inventors, earnest studies for solving the above problems, it has been found that by using specific compounds as a deforming agent, it is possible to produce a transparent and colorless aromatic polycarbonate having a high quality, at a high polymerization velocity without occurrence of severe foaming phenomenon. The present invention has been attained on the basis of the finding.